Field of the Invention
The present invention relates to a zoom optical system having an optical action that forms an image of a light flux emitted from one point on an enlargement conjugate side, in an intermediate imaging position, and then re-forms an image on a reduction conjugate side, and an image projection apparatus having the zoom optical system.
Description of the Related Art
Recently, a zoom optical system used in an imaging apparatus such as a digital camera or an image projection apparatus such as a projector has been required to reduce the size of an entire zoom lens, and have a wide angle of view and high precision optical performance over an entire zoom range. As the zoom optical system that satisfies these requirements, there is a zoom optical system employing an intermediate imaging technique (re-imaging method). In the intermediate imaging method, when an image of an enlargement conjugation point (object point) on an enlargement conjugate side (object side) is formed on a reduction conjugate side (image side), the image of the enlargement conjugation point is formed once in an intermediate imaging position in the zoom optical system, and then the image is re-formed in a reduction conjugation point (see U.S. Pat. No. 7,283,309, U.S. Pat. No. 7,791,816, and US2014/204351).
U.S. Pat. No. 7,283,309 discusses a zoom lens system configured to form a first intermediate real image of an object by a first zoom group, and form a final image based on the first intermediate real image by a second zoom group.
U.S. Pat. No. 7,791,816 discusses a zoom lens configured to form a primary image of an object by a first lens group, and re-form the primary image on an imaging surface by a relay lens having a zoom function. The relay lens in U.S. Pat. No. 7,791,816 includes, in order from an object side to an image side, an R1 unit with positive refractive power, an R2 unit with positive refractive power, and an R3 unit with negative refractive power, and performs a zoom operation by moving the R1 unit and the R2 unit.
US2014/204351 discusses a projection optical system used in a projector apparatus and including a first refractive optical system having a zoom function and configured to form on an enlargement side a first intermediate image of an incident light from a reduction side, and a second refractive optical system configured to form an image of the first intermediate image as a second intermediate image on the enlargement side. Furthermore, the projection optical system includes a first reflective optical system with positive refractive power and projects the second intermediate image to a screen on the enlargement side. The first refractive optical system includes a first lens group G1 with positive refractive power, a second lens group G2 with positive refractive power, and a third lens group G3 with positive refractive power, and performs a zooming operation by moving the first lens group G1 and the second lens group G2.
When the re-imaging method is used in the zoom optical system, a viewing angle (angle of view) can be easily widened, and the size of the entire zoom lens can be easily reduced. Especially, when the re-imaging method is used in the zoom optical system (imaging optical system) used in the image projection apparatus (projector) which magnifies and projects an image provided by a liquid crystal display (LCD) element, on a screen surface, the size of the entire zoom lens can be reduced, and the image can be projected with a large projection angle. However, according to the zoom optical system using the re-imaging method, despite the above characteristics, the aberration considerably varies at the time of zooming, and it is difficult to obtain high optical performance over an entire zoom range.
Especially, when the viewing angle is widened, aberrations such as image distortion and excessive curvature of field considerably vary at the time of zooming. Therefore, in the zoom optical system using the re-imaging method, it is important to appropriately set a lens configuration, the number of lenses (or lens groups) moving at the time of zooming, the refractive power of each lens group, a moving condition, and an intermediate imaging position in a light path. When these parameters are not appropriately set, the aberration variation is increased at the time of zooming, it is difficult to reduce the size of the zoom lens, the viewing angle cannot be widened, and high optical performance over the entire zoom range cannot be obtained.
U.S. Pat. No. 7,283,309 discusses a zoom lens having five moving lens groups in which lens groups moving at the time of zooming are provided in each of a first zoom group and a second zoom group. When the zooming operation is performed using the first zoom group, the aberration and the curvature of field are likely to considerably change in a wide-angle end and a telephoto end.
In the zoom lens discussed in U.S. Pat. No. 7,791,816, only two lens groups move at the time of zooming in a relay lens, and the number of the movable lens groups is small. However, the positive refractive power of each of the two lens groups is strong, so that the aberration is likely to considerably vary at the time of zooming. Furthermore, the zoom lens does not form a telecentric optical system on the image side, so that it is difficult to apply the zoom lens to a projector or the like.
According to US2014/204351, while the zooming operation is performed by moving two lens groups each having positive refractive power, the aberration is likely to considerably vary at the time of zooming.